North Korea provoked ferocious condemnation last year when it carried out several missile launches and nuclear tests.
Donald Trump responded by promising “fire and fury” before backing down an issuing sanctions.
Earlier he again promised economic warfare by promising the “largest ever” raft of sanctions on the hermit state.
But this evening he again returned to a more aggressive stance, warning North Korea to respond sensibly to sanctions or face “phase two”.
Mr Trump did not specify what this meant but his response implied a forceful response which moved beyond economic sanctions.
He said: “We’ll have to see. If the sanctions don’t work, we’ll have to go phase two.
“Phase two may be a very rough thing, may be very, very unfortunate for the world. But hopefully the sanctions will work.”
Earlier today he said North Korea will face the “largest ever” raft of sanctions.
The US Treasury’s Office of Foreign Assets Control today formerly announced the measures, which are designed to disrupt North Korean shipping and trading companies and vessels and to further isolate Pyongyang.
The Treasury also sanctioned one person, 27 companies and foreign 28 ships for breaking sanctions on North Korea.
The ships are located, registered or flagged in North Korea, China, Singapore, Taiwan, Hong Kong, Marshall Islands, Tanzania, Panama and Comoros.
It comes after North Korea launched several missiles directly over Japan, a major US ally, last year, sparking fury.